1. Field of the Invention
This invention relates to the recovery of mineral values from subterranean formations and more particularly to a process for leaching subterranean mineral deposits to recover the mineral values.
2. Description of the Prior Art
The in situ leaching of mineral values from subterranean deposits is well known in the art as a practical and economical means for recovering certain elements such as uranium, copper, nickel, molybdenum, rhenium, vanadium and the like. Basically, solution mining is carried out by injecting into the subterranean deposit a leaching solution which will solubilize the mineral value desired to be recovered and the solution and solubilized mineral values are recovered from the deposit for subsequent separation of the mineral values. Often it is necessary to oxidize the mineral value to a form where it can form a soluble reaction product in the leaching solution. Depending upon the nature of the subterranean deposit, the typical leaching solution may be an acid, for example, an aqueous sulfuric acid solution or may comprise an alkaline carbonate solution. In view of the high proportion of carbonates typically present in many subterranean formations, the use of acid solution is usually prohibitive because of the excessive consumption of acid due to carbonate solubilization. Consequently, alkaline carbonate leaching solutions are preferred to acid solutions for solution mining operations when carbonates are present in the formation.
Various solution mining processes involving the use of alkaline carbonate leaching solutions are disclosed in the prior art, for example in U.S. Pat. No. 3,708,206, using an aqueous ammonium carbonate solution and an oxidizing agent and U.S. Pat. No. 2,896,930, utilizing an aqueous solution of an alkali metal carbonate and hydrogen peroxide. Typically, the alkaline carbonate leaching solutions are maintained at a pH of about 8.2 to about 8.3. However, alkaline carbonate leaching solutions can present problems with respect to reduction of injectivity of the leaching solution and consequentially reduced mineral production. Reduced injectivity can generally be ascribed to two major factors. The first factor is the swelling of formation clays by the alkaline solution. A second major factor is the exchange of cations, such as sodium, from the leaching solution with the calcium ions of the formations which results in the production of calcium carbonate in the leaching solution. Calcium carbonate, at the alkaline pHs utilized, precipitates from the leaching solution thus reducing the permeability of the formation, well injectivity, and causes scaling problems in the pumps and other ancillary equipment utilized in the process.
Another problem which arises from the use of the prior art alkaline carbonate leaching solutions, particularly the ammonium carbonate and/or ammonium bicarbonate leaching solutions, involves the restoration of the ground water in the subterranean formation to its preleach condition once the solution mining operation has been completed. The cations introduced into the formation by these leaching solutions are ionically exchanged into the formation matrix and must later be removed. The removal of these exchanged cations is often difficult and, in the case of the ammonium ion, has been found to require a prohibitively long restoration period.
The present invention overcomes the foregoing deficiencies by providing a process for the recovery of mineral values from subterranean formations in which the loss of permeability and production associated with the alkaline leaching solutions is avoided.